Different kinds of particulate medium-containing devices have been long used for treatment of wastewater. They include static bed reactors and fluidized bed reactors. Static bed reactors comprise subsets which include trickling filters and depth filters. The systematics of devices varies in the literature; the term packed bed filters is sometimes used to encompass both types of filters; at other times, the term designates depth filters in distinction to trickling filters. Generally, trickling filters employ large size medium, often comprised of specially shaped ceramic and plastic pieces; and they are designed so that air or other gas can easily flow, usually by convection, around the medium, over which wastewater trickles downwardly.
The present invention is concerned with downflow depth filters, often called sand filters, which as the name connotes, comprise fine particulate medium, e.g., common sand. Insanitary engineering terms, sand is typically defined as having a granular medium having particle size between 0.15 and 2 mm. Since the pores between the sand grains are small, sand filters typically are substantially saturated with water during use, and there is negligible convective air flow within the bed.
Sand filters have been long used for removing suspended solids from, or for clarifying wastewater and other liquids, such as wastewater which flows from a septic tank or other source. Typically, a downflow type sand filter comprises a mass of sand, commonly called the sand bed, which has a depth of two to six feet. Water is sprayed or otherwise distributed upon the surface of the bed on a continuous or intermittent basis, and flows through the bed by force of gravity.
A sand filter may be created within a cavity in the natural earth. Sometimes, the wastewater flows through the sand and directly into underlying soil, in which case the device is sometimes called a bottomless depth filter. Often the medium of the filter bed is contained within a concrete, metal or plastic container, and drains at the bottom of the bed collect the water, so it can be flowed elsewhere. Drains may also be used with filters created within a cavity within the earth. Sand filters vary widely in physical size, in particular in surface area. Small cross section filters may be used for domestic and light industrial wastewater. Large filters may be used in municipal treatment plants.
One aspect of sand filter operation is mechanical: suspended solids in the wastewater fetch up in the interstices of the medium, as the water wends its tortuous downward path. Another anticipated effect is biochemical, wherein, as a result of facultative microbiological decomposition, it is anticipated there will be a reduction in Biological Oxygen Demand (BOD) of the wastewater, conversion of harmful constituents to more benign forms.
According to wastewater engineering principles, effectiveness of sand filters over time is dependent on proper design and operation. Generally, the biochemistry, volume and frequency of dosing of the inflowing wastewater must correlate with the size and other characteristics of the filter. Frequent maintenance, commonly backwashing, is called for. How such principles are adhered to in practice is always a question. It is known that conventional sand filter beds often present operational problems. A major problem is that a filter becomes clogged or “sluggish” with respect to through-flow of wastewater. That is, the filter tends to progressively becomes less accepting of the design-intended wastewater flow; the apparent hydraulic conductivity is decreased. The water head loss through the filter increases, and by and by a pond of water will appear on the top surface of the bed, even though inflow has not been changed. Ultimately, if no remedial action is taken, the system performance degrades to the point that the system fails to fulfill its intended purpose.
Clogging and associated ponding are normally attributed to an accumulation of solids upon the bed and within the interstices of the bed. Frequently, a layer or mat of foreign matter may be observed on the sand bed surface. Of course, that is not surprising since the bed surface might be expected to act as a primary mechanical strainer of suspended solids. Once a mat starts to form, there is an accelerating trend toward failure.
Various techniques are commonly used to avoid or counter the foregoing kinds of degradation. In one common approach, the bed is backwashed with up-flowing water, sometimes abetted with an upward air scour. The idea of backwashing is to disrupt the medium, so that clogging matter is carried away with the backwash water. In another remedial action, the bed surface is mechanically raked to disrupt the mat. However, raking requires good access to the bed surface, can be distasteful for a worker, and the process can allow portions of the mat to move deeper within the bed. In another approach, the top layer of the medium, or the whole of the medium, can be replaced. While these alternative maintenance activities can be effective, it is found that they often need to be done frequently, can be temporarily disruptive to continued wastewater processing, may not be wholly, effective, or may be costly. Thus, improvements are sought.